1. Field of the Invention
The present invention relates to an LC voltage-controlled oscillator (VCO), and more particularly to an LC VCO capable of relaxing restrictions on a power supply voltage level required for oscillation and oscillating without a varactor device.
2. Discussion of Related Art
A VCO is a circuit whose oscillation signal can vary in frequency according to a voltage applied from the outside, and is used as an important component in a wireless transceiver.
Among VCOs, an LC-type VCO uses negative resistance according to a positive feedback of a circuit. The oscillation signal of such an oscillator can be controlled by controlling a capacitance value of a varactor device present in the circuit using a control signal.
As an LC-type VCO, a negative conductance LC oscillator using a negative resistance characteristic based on a positive feedback of a transistor is widely known.
FIG. 1 is a circuit diagram of a general LC VCO.
As shown in FIG. 1, a general LC VCO includes an LC resonant circuit 110 including at least one inductor L1, a capacitor C5 connected in parallel with the inductor L1, and variable capacitors C2 and C3 included in at least one varactor device, and an amplifier circuit 120 having a positive feedback circuit including two transistors M1 and M2 whose gates and drains are connected, and a current source circuit 130 having a current source I1.
Also, both terminals of the inductor L1 and the variable capacitors C2 and C3 connected in series are connected to output nodes outp and outn, and the drains of the transistors M1 and M2 included in the amplifier circuit 120 are connected to the output nodes outp and outn respectively.
The LC VCO oscillates when an absolute value |Rin| of an input impedance Rin=−2/gm of the positive feedback circuit constituting the amplifier circuit 120 is an equivalent resistance of the LC resonant circuit 110 or less. The oscillation frequency, of an output signal can be expressed by Equation 1 below. Here, CT is a combined capacitance value of the capacitor C1 and the variable capacitors C2 and C3.
                              f          osc                =                  1                      2            ⁢            π            ⁢                                                            L                  1                                ·                                  C                  T                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
As seen from Equation 1, the oscillation frequency of an output signal of the VCO varies according to the inductance value of the inductor L1 included in the LC resonant circuit 110 or the combined capacitance value CT. In general, a spiral inductor consisting of a spiral line and an outgoing line is used as the inductor L1 and formed on the same substrate as the transistors M1 and M2. Here, the inductance value of the inductor L1 varies differentially, and it is very difficult to control the oscillation frequency by adjusting the inductance value. Thus, a fixed value is used as the inductance value of the inductor L1, and a method of adjusting the capacitance values of the variable capacitors C2 and C3 by applying a control signal vc to the variable capacitors C2 and C3 constituting the varactor device is widely used to control the oscillation frequency. Here, the variable range of the capacitance value of the varactor device corresponds to the variable range of the oscillation frequency.
Meanwhile, for operation of the VCO shown in FIG. 1, a voltage for driving the current source I1 of the current source circuit 130 and a voltage for driving the positive feedback circuit included in the amplifier circuit 120 are required. In other words, the sum of the two voltages is required as the minimum power supply voltage for operation of the VCO of FIG. 1. Thus, the current source I1 needs to be removed, so that the VCO can be driven even at a low power supply voltage.
FIG. 2 shows the VCO of FIG. 1 from which the current source I1 has been removed for this reason. Here, the gate-source voltage of the transistors M1 and M2 needs to be a threshold voltage or more, so that negative resistance can be obtained by positive feedback in the amplifier circuit 120. However, in the VCO of FIG. 2, a power supply voltage VDD corresponds to the gate-source voltage of the transistors M1 and M2, and thus needs to be the threshold value of the transistors M1 and M2 or more.
Also, the oscillation signals of the VCOs shown in FIGS. 1 and 2 are adjusted by a varactor device, which may restrict characteristics of the VCOs. Further, it is impossible to design the VCOs without a varactor device.